Generally from what I've seen, there are only two ways a key gets revoked as a result of key sharing.
1) Posting on the forums that you gave a particular key to someone else.
2) The key in question being abused by the people you give it to, or it being passed around by a number of people and it getting deactivated automatically for being in use in too many different places.
It's good to keep in mind that your authentication key is basically a permanent, unchangeable password to your account that gives absolute access to your account and identity on Blockland. Unless the key is specifically intended for them, if you give it to someone you must have complete trust that they will not abuse it or give it to other people. Protip: You can't really trust people not to do that. Just tell them to buy a key or buy a key for them.